Electric utilities use alternating-current (AC) power systems extensively in generation, transmission, and distribution. Most of the systems and devices involved operate on three-phase power, where voltages and currents are grouped in threes, with the waveforms staggered evenly. The basic mathematical object that describes an AC power system waveform (current of voltage) is the “phasor” (phase angle vector).
Devices known as Phasor Measurement Units (PMUs) have been commercialized by several companies to calculate phasors from power waveforms. Because phase angle is a relative quantity, it is necessary when combining phasors taken from different parts of a power grid to align the phase angle elements to a common phase reference; this is done in PMUs through the use of GPS timing signals. Such phasors are known as synchrophasors. PMUs measure synchrophasors, but existing devices use phasor calculation methods that have a number of shortcomings. For example, in addition to being generally expensive and burdensome to embedded processors (and thus mostly deployed in transmission grids rather than distribution grids), PMUs generally only calculate the positive sequence, even for unbalanced phasors, but all three sequences are needed for many applications. Furthermore, PMUs use batch-type calculations to produce phasor values, and are thus inadequate to provide phasors on a sub-cycle basis, such as for various protection applications. Moreover, today's synchrophasor calculation methods generally introduce various errors, such as phase lag and group delay, and do not account for variance in actual power system frequency.